This invention relates to hydrogen sensors and more particularly to such sensors that produce a color change in the presence of hydrogen.
Fiber optic hydrogen sensors are available commercially. Such apparatus employ an optical fiber with a thin palladium or platinum film at the tip of the fiber. The thickness of the film changes in the presence of hydrogen. A light source directs light down the length of the fiber and a detector senses an interference pattern representative of the distance between the fiber tip and the surface of the film (i.e. the film thickness); a function of the presence of hydrogen. This type of apparatus is sensitive to the presence of from zero to one percent of hydrogen.
In another type of commercially available fiber optic hydrogen sensor, an optical fiber includes a Bragg grating with a palladium film deposited over the grating. The fiber is stretched and in the presence of hydrogen, the palladium film affects the characteristics of the fiber to alter the wavelength of light reflected by the grating. In use, a light source, launching light down the fiber, produces a reflected wave at a wavelength representative of the presence of hydrogen. The greater the hydrogen concentration, the greater the change in the grating wavelength. This apparatus is sensitive to the presence of hydrogen from zero to ten percent concentration.
In a third known apparatus, a film of palladium is formed at the end of a fiber with a tungsten oxide coating formed over the palladium. In the presence of hydrogen, the palladium film places a charge on the tungsten oxide that changes its color.
Each of these commercially available apparatus is typically fabricated with an external thin metal film at the termination of the fiber that is susceptible to contamination, scratching, and other environmental factors. As a result, the sensors are limited in sensitivity, relatively expensive to manufacture, have a limited lifetime, and can produce unreliable results.
In accordance with the principles of this invention, one embodiment of the sensor comprises a hydrogen sensing element of porous silica glass, having a matrix of tungsten oxide and palladium in energy coupled proximity, fabricated in the form of a tiny rod, typically of cylindrical geometry, with a diameter of three millimeters and a length of five millimeters. The sensor can also be fabricated in other forms including, but not limited to, rectangular, square, and disk formats. The porous glass rod is doped with a solution of a photosensitive tungsten hexacarbonyl (W(CO)6) compound in an alcohol solvent to entrap the compound in the glass. The rod is next exposed to ultraviolet light, to induce a photochemical reaction transforming the tungsten hexacarbonyl compound into tungsten oxide (WO3) and permanently binding the tungsten oxide to oxygen atoms in the silica glass. A heating process drives off the solvent and removes the photoproduced CO gas, leaving tungsten oxide permanently bound to the silica matrix of the porous glass. The rod is then doped with a palladium tetrachloride (PdCl4) solution, and heated to break the PdCl4 molecules into palladium metal and chlorine gas and to evaporate the chlorine gas from the porous glass matrix, leaving palladium metal deposited on the glass surface.
When hydrogen is present, the Pd/WO3 complex causes a partial redox reaction of the tungsten oxide and palladium metal to produce a color change in the glass rod representative of the hydrogen concentration. The rod may further include a coating to reduce the sensor""s sensitivity to moisture and other environmental contaminants.
The unique chemochromatic process of the invention binds tungsten oxide and palladium into the porous glass volume of the sensing element, thereby eliminating the need for external surface films. As such, the sensing chemistry is embedded in a relatively large, three-dimensional, pore volume, compared to that of a two-dimensional surface film, providing improved sensor sensitivity and increased immunity to contaminants that may be present on the surface of the sensing tip. In addition, the use of a wet chemistry process, instead of a thin film fabrication process, reduces manufacturing costs and produces a more rugged and reliable sensor structure.
In an apparatus of the invention, the sensing rod, prepared as above, is attached to the tip of a double fiber arrangement whereby light launched into one fiber is passed through the sensing rod, reflected from the back surface of the rod and launched into the second fiber where it is brought to an optoelectronic detection and signal processing unit. In the presence of hydrogen, the spectrum of transmitted light shifts, producing a color (wavelength) change that is transformed into an intensity change by the optoelectronic detection units. The invention is sensitive to hydrogen concentrations in the range of zero to one hundred percent with maximum sensitivity in the zero to five percent range and with a resolution of 0.1% hydrogen.
In another apparatus of the invention, a multi-point fiber optic sensor system (MFOS) is constructed using multiple sensing elements distributed along a communication bus. An optoelectronic readout system connected to the bus is adapted to display the concentration profiles of individual sensor readouts. The system can be used, for example, to monitor gas leaks in launch vehicle fuel tanks and can be distributed along automobile fuel cells.
In another apparatus of the invention, a sensor package is constructed with a hydrogen sensitive element and a temperature sensitive element in close proximity so that the signal returned from the hydrogen element can be compensated for temperature fluctuations.
In another embodiment, the sensing element can be in the form of a porous glass film or layer treated as the herein described matrix of tungsten oxide and palladium in energy coupled proximity for hydrogen sensing and which can be applied to a supporting substrate.